In the parent, if the fork call succeeds, $pid will contain the process ID of the child. You can then use the kill command to send any sort of signal to it (as you know the PID).

my $pid = fork();
if ($pid == 0) {
    # we're the child
    # exec something or other
} else {
    sleep 10;    # give the child a chance
    kill 1, $pid;    # nighty-night
}
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Something along those lines will suffice.